This invention relates to a process and an apparatus for shifting the water gas shift reaction (CO+H2OCO2+H2) towards the production of carbon dioxide and hydrogen by adsorbing carbon dioxide produced by the reaction.
A conventional practice for producing hydrogen product containing low levels of carbon oxides (carbon dioxide and carbon monoxide) is to purify raw syngas (from steam methane reformer, partial oxidation reactor, autothermal reformer, etc.) by cooling the gas to 600-950° F., reducing CO to ˜1-5% in a high temperature shift (HTS) reactor, cooling the gas to 350-600° F., further reducing CO to ˜0.2-0.4% in a low temperature shift (LTS) reactor, cooling the gas to 100° F., removing CO2 in a liquid chemical or physical absorption system, and methanating carbon oxides. A more recently developed industrial practice comprises cooling the gas to 600-950° F., reducing CO to ˜1-5% in a high temperature shift (HTS) reactor, cooling the gas to 100° F., and removing CO2, CO, CH4 and N2 in a H2PSA unit.
The shift reaction is carried out in shift reactors. These reactors are used to increase the amount of H2 produced from the process and reduce the level of CO in the feed gas to the separation unit. The reactors are designed to permit very close approach to reaction equilibrium, so the CO conversion is limited by the shift reaction thermodynamics. Both the reaction temperature and the presence of byproduct carbon dioxide influence the reaction conversion.
Since the shift reaction (CO+H2O—>CO2+H2) is exothermic, the CO conversion is increased with lower temperature. High temperature systems, utilizing only HTS reactors, are limited to ˜75% CO conversion, and the rest of the CO (and H2 that in principle could be produced via the shift reaction) is lost. The overall CO conversion can be increased to >95% by utilizing a series of HTS and LTS reactors at the expense of additional process complexity (additional cooling and LTS equipment).
Since the shift reaction conversion in conventional reactors is not 100%, a significant amount of CO will be present in the shift reactor exit gas. The effluent gas from a typical LTS reactor contains roughly 0.3% CO (3,000 ppm), 19.5% CO2, 1.3% CH4, and 78.9% H2 (dry basis). Effluent gas from the HTS reactor will contain even more CO (1-5%) and less CO2. The effluent gas must be further purified, typically in a 4- to 12-bed H2PSA unit, to reduce the CO and CO2 levels to <0.01%. In PEM fuel cell applications, the CO level must be reduced even further (limits typically around 10 ppm).
The presence of CO2 in the HTS and LTS reactors limits the CO conversion that can be achieved. If CO2 could be removed from the feed gas to the shift reactors, the CO conversion would be increased. This is not practiced, though, since the gas would require cooling for CO2 removal, followed by reheating for shifting, and the added complexity is not worthwhile. Ideally, one would prefer to remove CO2 from the shift reactor feed gas, and also remove CO2 from the reaction gas as it progresses through the shift reactor. If CO2 can be removed completely as it is formed, then the CO conversion in principle could reach 100%, and the CO would be reacted to extinction.
Accordingly, there have been a number of efforts to shift the water gas shift reaction towards production of carbon dioxide and hydrogen (i.e., towards completion) by adsorbing carbon dioxide produced by the reaction. A number of these efforts have comprised adsorbing CO2 on chemical adsorbents such as calcium oxide or dolomite. Regeneration of these materials is possible only by heating the solid to 750° C. or higher, so generally these processes are classified as temperature swing adsorption systems.
For example, U.S. Pat. No. 1,816,523 to Gluud et al. proposed the use of lime or dolomite to remove CO2 from the shift reaction, and regenerated the carbonate by burning fuel in the vessel to increase the temperature to 900-1000° C.
Han et al., “Simultaneous Shift Reaction and Carbon Dioxide Separation for the Direct Production of Hydrogen”, Chem. Eng. Sci., 49, 5875 (1994), and “Multicycle Performance of a Single-Step Process for H2 Production”, Sep. Sci. Tech., 32, 681 (1997) have also worked with the same chemisorbent, and found that the CO2 capacity and carbonation rate of the dolomite decreased as it was cycled over a number of reaction/regeneration steps. In their 1994 publication, Han et al. found that the major economic obstacle for producing H2 from coal using this approach was due to the substantial regeneration energy requirement.
Others have tried to use dolomite or calcium oxide chemisorbents in a reformer to enhance the steam methane reforming reaction. This approach is again based on the fact that removal of CO2 from the reactor shifts the water gas shift reaction, which in turn will shift the reforming reaction to higher conversion.
For example, Brun-Tsekhovoi et al., “The Process of Catalystic Steam-Reforming of Hydrocarbons in the Presence of Carbon Dioxide Acceptor”, Proc. 7th World Hydrogen Energy Conf., 2, 885 (1988) and Kurdyumov et al. “Steam Conversion of Methane in the Presence of a Carbon Dioxide Acceptor, Pet. Chem., 36, 2, 139 (1996) describe fluidized bed processes, wherein Ni reforming catalyst and dolomite are fluidized with steam and natural gas. The authors observed increased methane conversion, relatively high H2 purity (94-98%), and low carbon oxide levels. They mention that the process may be capable of eliminating the need for CO-shift, CO2 removal, and methanation steps, and could reduce the required operating temperature of the reformer. The articles teach the use of catalyst and dolomite of different sizes to allow separation of spent dolomite, which is subsequently regenerated at high temperature in an external furnace.
WO 96/33794 (Lyon) discloses a somewhat similar approach, using a fixed bed, CaO and Ni catalyst. The reaction of steam and hydrocarbon is carried out at 600-800° C., and the CO2 formed reacts to form CaCO3. Passing air through the bed regenerates it. The O2 in the air exothermically reacts with Ni to form NiO, and the energy from this reaction is used to decompose CaCO3 to CaO and regenerate the chemisorbent.
Researchers at Air Products and Chemicals, Inc. have obtained patents on a Sorption Enhanced Reaction process, wherein a high temperature CO2 adsorbent is used to remove CO2 and shift the steam methane reforming reactions to higher conversions. See, e.g., U.S. Pat. No. 6,303,092 to Anand et al. and U.S. Pat. No. 6,315,973 to Nataraj et al. and the related publications, Carvill et al., “Sorption Enhanced Reaction Process”, AlChE J., 42, 2765 (1996), Hufton et al., “Sorption Enhanced Reaction Process for Hydrogen Production”, AlChE J., 45, 248 (1999) and Waldron et al., “Production of Hydrogen by Cyclic Sorption Enhanced Reaction ”, AlChE J., 47, 1477 (2001). These researchers have described a process used to shift the reverse water gas shift reaction to enhance CO production by adsorbing water on a high temperature water adsorbent. A specific process cycle is described which utilizes pressure swing adsorption concepts for regeneration of the adsorbent. The use of CO2 adsorbents for shifting the reforming reaction is also described.
Air Products and Chemicals, Inc. researchers have also patented high temperature CO2 adsorbents particularly suitable for use in the aforementioned processes. See U.S. Pat. No. 6,280,503 to Mayorga et al.
EP444987 to Ogawa et al. discloses the application of high temperature CO2 adsorption processes for removing CO2 from turbine feed gas. Ogawa et al. used a shift reactor with heat removal to shift CO to CO2, cooled the gas, and then passed it through a separate CO2 PSA unit at 200-300C to remove CO2. The shift reactor effluent contains all of the carbon present in the reformer feedstock. The goal was to remove carbon from the fuel gas before it was fired in the gas turbine. Specific adsorbents or adsorption process cycles are not disclosed.
Despite the foregoing developments, it is desired to provide a system of enhanced efficiency for shifting the water gas shift reaction towards the production of carbon dioxide and hydrogen by adsorbing carbon dioxide produced by the reaction.
All references cited herein are incorporated herein by reference in their entireties.